A rotor comprising a vertical rotation axis and at least two rotor blades arranged on the rotation axis, wherein at least one rotor blade comprises a first portion and a second portion, wherein the second portion is moveably arranged on the first portion, may be used to generate energy from water, air or other fluid flows. In the operation of such a rotor, at least one rotor blade moves in the direction of or together with the fluid flow and at least one rotor blade moves against the direction of the fluid flow or against the fluid flow. A wind motor is disclosed for example in FR822092, wherein the rotor blades comprise a vane and a flap element which is moveably arranged at the vane. Under normal wind conditions, the flap is positioned on a back side of the vane, but under extreme wind speed, the flap swings out of the back side of the vane. This way, a back pressure is created between the vane and the flap, so that the wind motor is decelerated to avoid damage due to extreme wind speed.